Barcelona

David Ramos/Getty ImagesLionel Messi and Christian Tello have been key figures for Barcelona this season

Barcelona remain the potent threat to every other side in the Champions League tournament.

While their qualification from Group G was not as straightforward as many anticipated, a safe passage to the last 16 was comfortably negotiated in the end.

Despite the shock defeat at Celtic and having to come from behind to claim a 3-2 win at Spartak Moscow, the Catalan giants progressed with a game to spare.

Lionel Messi, who scored five times in the group stage, remains the key player for Barcelona, but theemergence of Christian Tello in the competition has also been important.

The 21-year-old forward has improved with every appearance and is gaining the look of an accomplished finisher.

New contracts for both Messi and Tello underline the impact the youngster has made with the club sincescoring twice within eight minutes of his Champions League debut as a substitute against Bayer Leverkusen last season.

The pain of defeat in the semifinals to Chelsea last season still rankles with Barcelona supporters,but it is the heartache of the Nou Camp players which will act as their spur this season.

Real Madrid

Jasper Juinen/Getty ImagesCristiano Ronaldo will be a crucial player as Jose Mourinho targets Champions League triumph for Real Madrid

Real Madrid have not won the Champions League since 2002, a statistic which haunts the Bernabeu Stadium with La Liga rivals Barcelona picking up the plaudits instead.

The heady days of Los Galacticos may be behind Real, but their squad remains a potent force under manager Jose Mourinho.

Mourinho knows how to win the Champions League after succeeding with FC Porto and Inter Milan but recognises nothing less will be good enough in his current role.

But Real were not consistent in the group stages, needing late goals to defeat Manchester City 3-2 at the Bernabeu in their opening match and took only one point from their matches with Borussia Dortmund.

The German side topped Group D, leaving Real among the second tier of seeds for the last-16 draw.

That is unlikely to concern Mourinho, who has players such as Cristiano Ronaldo at his disposal.

The 27-year-old is the top goal scorer in this season’s Champions League with six from six games and, like his Barcelona counterparts, was badly hurt by the semifinal loss to Bayern Munich last season.

Manchester United

Clive Rose/Getty ImagesRobin van Persie has been an inspiration for Manchester United in the Champions League this season

Nobody at Manchester United will claim this has been a vintage campaign for the Old Trafford club.

Sir Alex Ferguson’s side might have qualified for the knockout stages with two games to spare in Group H, but they also fell behind in five of their group matches.

That statistic alone will be a warning that this may not be United’s season.

Nemanja Vidic will return to the side after the knee ligament injury he suffered against FC Basel a year ago saw him make just five appearances in the last 12 months.

Arguably, the absence of the 31-year-old Serbia international was one of the reasons United surrendered their English Premier League title to Manchester City last season.

But with Vidic now entering the latter stages of his career, it is open to debate whether the injury will affect his ability alongside Rio Ferdinand, 34, in a United side which managed just one clean sheet in Group H.

Thankfully for United, they possess an attacking prowess to envy with Wayne Rooney and Robin van Persie finding their groove together.

Van Persie has proved an astute buy for the Champions League this season, sparking a comeback at Braga after coming off the bench and scoring twice to overturn a deficit against CFR Cluj in Romania.

Bayern Munich

The pain of last season’s Champions League final defeat must have been excruciating for Bayern Munich.

The Bundesliga team had seen off the might of Real Madrid to reach the final in front of their own supporters at the Allianz Arena but then lost on penalties to Chelsea.

However, Jupp Heynckes’ side appear to have displayed little ill-effect from that heartbreak, starting their Group F campaign with a 2-1 win over Valencia.

A 3-1 defeat at BATE Borisov followed but that proved to be a mere blip as Bayern progressed to the knockout stages comfortably.

France international Franck Ribery remains the pivotal figure for Bayern even though he registered just one goal in the group stage.

His partnership with David Alaba on the left side of the Bayern team was a key factor in their successful group campaign.

Ribery scored just once in the group stages but Claudio Pizzaro and Thomas Muller have both struck three times, while Bastian Schweinsteiger and Toni Kroos added two each in the group stages as Bayern landed 15 goals in the Champions League this term.

But it is the drive of Schweinsteiger which really powers Bayern. The 28-year-old was rested for the game in Belarus against BATE and his absence was keenly felt in the surprise defeat.